Earlier today, Kai Schober has posted this on UO Herald:
Code activation has been re-enabled on the account center. You can once again enter game cards, item codes etc. in the account management under ‘Activate a Key’.
For your convenience we have created an account activation guide to explain the different options available on that page.
Please contact our customer support if you have additional questions.
So time to hit up UOGameCodes.com or find the emails from the past purchases you have made and haven’t had a chance to apply. From what I’m reading elsewhere, they all appear to be working fine.